There is an accumulation of black pieces in the g-file, but interestingly this isn't bad. With this move, Black takes advantage of the strength of the bishops and the support of the knight on g7 to break White's center.
20. f2e3  20... d5e4  21. f3e4  21... e6g4  22. c3e2 This move is refuted by Caruana's excellent reply. Both 22.Rc1 and 22.Rde1 keep the advantage.